Chlorine flavor perception and neutralization in drinking water

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Chlorine flavor perception and neutralization in drinking water

Sabine Puget

To cite this version:

Sabine Puget. Chlorine flavor perception and neutralization in drinking water. Psychology. Université de Bourgogne, 2010. English. �NNT : 2010DIJOS092�. �tel-00786522�

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UNIVERSITE DE BOURGOGNE

Ecole doctorale Environnements Santé STIC n° 490

Sciences de l alimentation

THÈSE

Pour obtenir le grade de

Docteur de l Université de Bourgogne Discipline : Sciences de l Alimentation

Présentée par Sabine Puget

le 7 mai 2010

Chlorine flavour perception and neutralization in drinking water

Directeur de thèse : Elisabeth GUICHARD

Co-directeur de thèse : Thierry Thomas-Danguin

Jury

Pr.Dr. Thomas Hummel, University of Dresden Medical School Rapporteur Dr. Catherine Rouby, Université Lyon 1, CNRS UMR 5020 Rapporteur Dr. Catherine Dacremont, Université de Bourgogne, Dijon Examinateur

Mr. Philippe Piriou, Suez environnement, Paris Examinateur

Dr. Elisabeth Guichard, INRA Dijon Directrice de thèse

Dr Thierry Thomas-Danguin, INRA, Dijon Co-directeur de thèse

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Acknowledgments

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---

This research work was funded by Lyonnaise-des-eaux et ANRT and carried out at the INRA UMR FLAVIC in Dijon under the supervision of Dr. Thierry Thomas-Danguin and Dr.

Elisabeth Guichard.

I acknowledge Elisabeth Guichard head of the UMR Flavic for welcoming me in her laboratory and giving me the opportunity to conduct a PhD in good conditions.

I acknowledge Thomas Hummel and Catherine Rouby and all the members of my PhD jury for giving me the honour to review my PhD work.

I gratefully thank my two PhD supervisors Elisabeth Guichard and Thierry Thomas-Danguin.

Thank you Elisabeth for your trust and the time you spent despite your responsibility. Thank you Thierry for your trust, encouragement and support, our fruitful discussions, the time you spent to review and improve my manuscript and finally your friendship.

I warmly thank the members of my PhD committee Didier Trotier and Boriana Atanassova for their interest in my work, our profitable meetings and their valuable advices and constructive criticisms.

I am grateful to Philippe Piriou and Virginie Roche, Lyonnaise-des-eaux, for their trust, time, and guidance to understand water supply and chlorination process.

I thank my colleagues for the project water good to drink Eric Teillet and Pascal Schlich for sharing with me their view on water perception.

I would like to thank Noelle Béno, Yoann Curé, Patricia Garnier, Nathalie Gondeau and Lucie Huault for helping me to achieve these crazy experiments with too many samples, for keeping their smile and motivation despite the workload

I would also thank Claire Chabanet for her help and guidance in statistics.

I would like to thank the members of the laboratory for their friendship and the good time we had, all the dinners we had, all the films we have seen together: Elodie, Akiko, Léri, Camille, Céline, Olivier, Marie, Charlotte, Gilles, Eric, Géraldine, Sylvie, Etienne.

I thank my family for giving me their support and encouragement.

Finally, I dedicate this thesis to my partner Jerome who is my constant supporter, my best ally and my love.

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General introduction

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General Introduction

In developed countries, opening water tap is a simple and daily gesture. The water supply does not constitute a problem in everyday life and water consumption is safe. However, it was not always the case in the past. In the 19^thcentury, waterborne infectious diseases remained frequent. The use of chlorine for water disinfection started in England by the end of the 19^thcentury after cholera and typhoid outbreaks. After demonstration of its efficiency, chlorination as disinfection process rapidly spread all over the world and permitted the control of disease-causing organisms. It constituted an essential step in ensuring water safety.

Nowadays, safety does not constitute a concern anymore and consumer s standards have changed. The taste of tap water became an important concern for consumers. They often report unpleasant taste and especially chlorine taste which is responsible for an important part of their complaints.

Some of the consumers may prefer the use of bottled water as drinking water, even if bottled water consumption would be associated with a higher economic and ecological cost (packaging, transportation). For consumers who resort to alternatives, tap water has a bad image. This explains why it also became an important concern for tap water suppliers.

Obviously, a solution to overcome chlorine taste of tap water would be to use chlorination alternatives such as ozonation or UV treatments. However, only chlorine-based disinfectants have residual properties. This property is of high interest since it allows a residual chlorine level being always present into the pipes and preventing micro-organism regrowth during water flow. This especially ensures microbiological quality of water from treatment plant to consumer s tap.

As a consequence, due to its residual properties but also to its low cost, chlorine is difficult to replace. A way to overcome chlorine taste problem in water would be to reduce its perception without reducing its actual concentration in water. Following this general idea, we explored, in the present Ph.D thesis, several ways to reduce chlorine flavour perception in water.

To address the question of water taste and chlorine taste, Lyonnaise-des-Eaux launched, in 2006, a project entitled Eau bonne à boire , in 2006. This project was developed in partnership with Vitagora^® which is the Taste-Nutrition-Health Competitive Cluster. The project has for objective to improve the taste of tap water in order to better satisfy French consumers. This project was divided into two parts. The first one relies on the mapping of consumer s preferences for intrinsic taste of water. E. Teillet was in charge of this component of the project and worked on this topic in the framework of his Ph.D conducted at the CESG (Centre Européen des Sciences du Goût). The second component of the project relies on the investigation of Chlorine taste perception and neutralization. This is the work presented in this thesis manuscript. It was done at the INRA UMR FLAVIC (FLAveur, VIsion &

Comportement du consommateur) in collaboration with the CESG, the CIRSEE (Centre - 8 -

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International de Recherche Sur l Eau et l Environnement) which is SUEZ ENVIRONNEMENT's international expertise center on water and the environment and finally, with Robertet, a company producing and distributing flavours and fragrances.

Before to develop the aims of the Ph.D work, it is important to give some general informations about perception. As aforementioned, some consumers complain about the unpleasant chlorine taste elicited during tap water consumption. From a scientific point of view, this chlorine taste perception experienced by consumer during water consumption does not rely on taste only, that is the interaction between tastants (chemicals) and the gustatory sensory system. The chlorine taste perception should more likely rely on a global percept called Flavour. Indeed, as defined by Beidler (1958, cited in Delwiche 2004) the term flavour rely on the sensation realized when a food or beverage is placed into the oral cavity. It is primarily dependent upon the reactions of the taste and olfactory receptors to the chemical stimulus. However, some flavours also involve tactile, temperature, and pain receptors. The 3 last sensations mentioned are conveyed by trigeminal nerve. Chemicals can also activate this nerve. More recently a review by Delwiche (2004) has highlighted that the different sensory modalities interact, especially within the chemical senses. Multi-sensory interactions especially take place during food and beverages consumption and lead to the formation of a single integrated percept, Flavour, which cannot be decomposed by simple introspection. Thus, during tap water consumption, one can experienced tactile and thermal sensation elicited by water flow in the mouth. Mineral presents in water could activate gustatory receptors located on the tongue. In the same way, chlorine could activate gustatory or olfactory receptors but also the chemosensory receptors located on the trigeminal nerve.

Nevertheless, the sensory mechanism involved in its perception remained unknown. In such context, our first aim was to determine the sensory modalities involved in chlorine flavour perception and to investigate the putative link between tap water rejection consumption and sensitivity to chlorine. Chapter 1 contains a short bibliographical review on chlorine flavour perception and its potential link with tap water consumption. This is followed by two publications on this topic and a short summary of the results.

On the basis of the results obtained in the first part of the Ph.D work, we then investigated the sensory interactions that could be associated with chlorine perception which could lead to a reduction of its perception. These aspects of the Ph.D work are presented in two chapters, each devoted to one specific approach. The first approach consisted in the determination of the potential masking effect of the different minerals presents in water. Indeed, as everyone could have experienced, water taste but also chlorine flavour intensity vary from one region to another. The explanation could lie on sensory interaction between perception elicited by the ions contained in water and chlorine flavour perception. We determined the ions varying in tap water distributed in France, their impact on water taste and finally their impact on

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chlorine flavour perception. This approach is developed in Chapter 2 which contains a short bibliographical review on water components that may elicit a sensory perception and their potential perceptive interaction with chlorine. After this bibliographical review, 4 publications are included and describe the experiments undertaken on this topic. A short summary of the results could be found at the end of the chapter.

The second approach followed in this part of the Ph.D work aimed to determine the potential interactions between selected odours and chlorine flavour. We especially tried to select odours that have the ability to reduce chlorine flavour perception. To avoid any perception of flavours in water that would likely be inadapted to tap water distribution, odorants were considered at sub/perithreshold concentrations. This work was partly performed in collaboration with Robertet S.A. and is presented in Chapter 3. This chapter contains a short bibliographical review on perceptual interactions between odours followed by two publications on this topic, and a short summary of the results.

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Chapter 1. Chlorine flavour perception

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Chapter I Chlorine flavour perception Introduction

1. Introduction

French public health code mentions that waters intended for human consumption have not to contain a number or a concentration of micro-organisms, parasites or other substances constituting a potential danger for the health of persons (de Forges et al. 2009). This type of regulations also exists in the US or in other European countries for example. Some of them defined requirements or guidelines concerning disinfectant residuals within the distribution system (Doré 1989; Connell 1996). For example, French Public Health Authority imposes a minimal value of 0.3 mg/L Cl2 at the treatment plant outlet and a minimum of 0.1 mg/L at tap (Journal Officiel 2001). Thus, chlorine addition is one of the most common treatments used to ensure tap water bacteriological quality. Chlorination had spread out all over the world due to its low cost and its efficiency. Indeed, it inactivates various types of micro-organisms such as Cryptosporidium, Gardia, but also various types of bacteria and viruses (Connell 1996;

Haas 1999; Haas and Engelbrecht 1980). Chlorination is usually ensured by gaseous chlorine (Cl2) inlet in water. Chlorine reacts with water to form hypochlorous acid (HOCl/ClO^- ). pKa of this acid has a value of 7.5.

Figure 1 : Evolution of the proportion of the different forms of chlorine according to pH

Since the pH value for water is usually comprised between 6 and 8, chlorine is mainly present in its associated form (HOCl) and dissociated form (ClO^-) into the pipes (Doré 1989).

It has been demonstrated that disinfection efficiency is higher when pH is lower, that is to say when associated form is present in greater quantity. One possible explanation lies on the negative charges coated on the surface of bacterial membranes. Thus, the associated form (HOCl) can penetrate more easily into the bacteria than the dissociated form (ClO^-). Thereby, - 12 -

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it alters enzymes and upsets internal machinery due to oxidation reactions (Connell 1996).

Due to its high reactivity, chlorine not only reacts with micro-organisms, but also reacts with chemicals present in water within the distribution system. These chemical reactions often lead to by products formation and off-flavours development (Froese et al. 1999; Heim and M. 2007). Specific tools such as odour wheel are available for targeting off-flavours in drinking water (Suffet and Rosenfeld 2007) and identify locations vulnerable to taste and odour problems (Proulx et al. 2007). However, chlorine by itself elicits a flavour which constitutes one of the major complaints advocated by consumers. This has been often noticed during various surveys all over the world (Chotard 2008; Miquel 2003; Suffet et al.

1996). Thus, 35 % of French consumers who participated in the SOFRES/C.I.Eau 2008 annual survey complained about water bad taste quality and especially the bad taste conferred to tap water by chlorine (Chotard 2008).

As far as chlorine flavour is concerned, several scientific publications investigated the link between tap water consumption, taste of water and risk perception associated with tap water consumption (Anadu and Harding 2000; Jardine et al. 1999; Levallois et al. 1999). Obviously, other factors are susceptible to affect water consumption behaviour such as cost of water or quality of raw water. Other publications were interested in chlorine flavour thresholds measurements. Bryan et al. (1973) were the first to measure chlorine thresholds. Theses authors measured flavour threshold of halogens and demonstrated that the chlorine flavour detection threshold decreases according to pH. Thus, threshold increases as pH and the hypochlorous acid dissociated form (ClO^-, non-volatile) increases. On the opposite, an increase of the volatile associated form (HOCl) is associated with a threshold decrease.

These observations suggest that it is the volatile associated form of chlorine (HOCl) which elicits chlorine sensory perception. However, chlorine detection mechanisms as well as sensory modalities involved in chlorine flavour perception remain unknown.

Chlorine perception threshold have been measured in various conditions. Krasner and Barrett (1984) determined that thresholds were 0.28 mg/L Cl2 for odour and 0.24 mg/L Cl2 for flavour. This means that olfaction at least is involved in chlorine flavour detection. Then, Piriou et al. (2004) compared chlorine flavour thresholds for different type subjects. These authors found a significant difference between a group of trained subjects (0.05 mg/L Cl2) and a group of untrained subjects (0.2 mg/L Cl2). They also compared the threshold in the US (1.1 mg/L Cl2) and in France (0.2 mg/L Cl2) and found a significant difference. The same year, Mackey et al. (2004a) measured chlorine flavour threshold in the U.S. and failed in finding a significant link between tap water consumption and chlorine flavour sensitivity.

However, it is noteworthy that only a few publications investigated the link between sensitivity to chlorine and water appraisal. This was done in Canada, by Turgeon et al. (2004). These authors demonstrated that consumers supplied with tap water containing a residual chlorine

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level greater than the threshold measured by Krasner and Barrett (1984) were less satisfied by tap water quality and perceived more risks. Data published up today did not clearly evidence a link between tap water consumption and chlorine flavour perception.

Therefore one important question remains to be answered: is chlorine flavour a determinant of tap water rejection?

In order to try to answer this question, we decided, to fully investigate the link between tap water consumption and chlorine flavour perception. Since chlorine perception mechanisms are not clearly elucidated, our strategy was not limited to the measurements of flavour thresholds. We also integrated the measurement of other hedonic and cognitive dimensions.

To do so, we compared the chlorine perception of two groups of consumers: a group including exclusive tap water consumers and a group including exclusive bottled water consumers. Sensory measurements performed with these two groups had for aim to compare their sensitivity but also their perception of chlorine through measurements including intensity, preference and acceptability. This work is detailed in a first paper (publication 1). In a second publication, we present a series of data obtained with the same consumers. This part of the work aims to determine the sensory modalities involved in chlorine flavour perception and their respective activation threshold values.

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Chapter I Chlorine flavour perception Publication 1

2. Publication 1:

Tap water consumers differ from non-consumers in chlorine flavour acceptability but not sensitivity.

Sabine PUGET^1,2, Noëlle BENO², Claire CHABANET², Elisabeth GUICHARD², Thierry THOMAS- DANGUIN^2,*

1 Lyonnaise des Eaux, 11 Place Edouard VII, F-75009 Paris, France.

2 Unité Mixte de Recherches FLAVIC, INRA, ENESAD, Université de Bourgogne, 17 rue Sully, BP 86510, 21065 Dijon Cedex, France.

* Corresponding author: Tel: +33 380693084; fax: +33 380693227; e-mail: Thierry.Thomas- [email protected].

This manuscript has been published in Water Research (2010), Doi:10.1016/j.watres.2009.10.009.

1. Introduction

Adding chlorine to tap water is one of the most common treatments to ensure its bacteriological quality. Used for the first time in England in the 1880s, chlorine treatment of drinking water has spread all over the world. Chlorine inactivates various types of micro- organisms and its residual properties help to prevent micro-organism regrowth during water flow in the pipes (Connell 1996; Al-Jasser 2007). Indirect sensory effects of water chlorination, linked to the development of off-flavours due to by-products, have been studied (Froese et al. 1999; Heim and M. 2007). Methodological developments were proposed to identify locations vulnerable to taste and odour problems (Proulx et al. 2007) and specific tools (e.g., the odour wheel) are now available for targeting off-flavours in drinking water (Suffet and Rosenfeld 2007; Burlingame et al. 2007; Suffet et al. 1988). Beyond off-flavours development due to chlorination by-products, chlorine flavour by itself constitutes one of the major complaints against tap water. In 1996, chlorine taste was the third most reported taste default of tap water in the US (Suffet et al. 1996). In France, an annual survey performed in 2008 by SOFRES/C.I.EAU (Chotard 2008) indicated that 40% of the interviewed consumers reported an unpleasant water taste and 34% an unpleasant chlorine taste. Due to the unpleasant taste of tap water, consumers may prefer bottled water as drinking water, even if bottled drinking water consumption would be associated with a higher economic and ecological cost (Milmo 2006). The 2008 SOFRES/C.I.EAU survey showed that 41% of consumers mainly drink bottled water and 26% are exclusive consumers of bottled water (Chotard 2008). In Canada, Levallois et al. (1999) also found that bottled water consumption

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was mainly due to organoleptic reasons. Turgeon et al. (2004) showed that the perception of tap water quality is closely related to the residual chlorine level: people living near a treatment plant who may receive a higher chlorine level in their tap water were generally less satisfied by tap water quality and perceived more risks associated with it than people living far from the plant. It was reported that, in the US, bottled water drinkers have three main categories for decisions: safety of water; healthfulness of the water; and taste of the water (Mackey et al. 2004^a,b). Consumers supplied with tap water containing a residual chlorine level greater than 0.24 mg/L Cl2 were less satisfied with tap water when compared to consumers receiving lower concentrations (Turgeon et al. 2004). This chlorine level likely corresponded to a chlorine detection threshold, as measured by Krasner and Barrett (1984).

When taken together, these studies underline the role of chlorine flavour in the lack of tap- water acceptance by consumers. Piriou et al. (2004) showed that chlorine detection thresholds in water vary according to subject experience, the threshold for experienced subjects being lower. In the same way, French consumers thresholds were lower than American ones, revealing cultural differences. Flavour is defined as a sensory percept induced by food or beverage tasting. It relies mainly on the functional integration of information transmitted by the chemical senses: olfaction, gustation, oral and nasal somatosensory inputs (Thomas-Danguin 2009). Once in the mouth, volatile compounds are retronasally conveyed to the nasal cavity where they are inclined to activate the olfactory receptors located on the top of the olfactory cleft and the trigeminal fibers inserted into the whole nasal mucosa. Soluble compounds could be dissolved into the saliva and some of them could further be detected by the gustatory cells of the taste buds and the trigeminal fibers also inserted in the oral mucosa (Laing and Jinks 1996; AFNOR 1992). These three sensory modalities are simultaneously activated and interact to create an integrated unique perception, flavour, which cannot be decomposed by simple introspection. As far as chlorine flavour is concerned, the mechanisms of perception remain poorly investigated. If chlorine flavour detection threshold in water has been evaluated several times, it is not however known whether this threshold, which reflects a subject s (or a group of subjects ) mean sensitivity to chlorine, is different for consumers who regularly use tap water as drinking water as compared to consumers who prefer bottled water. In summary, the bibliographical review showed that, on the one hand, chlorine flavour constitutes one of the major complaints against tap water. On the other hand, through available consumers inquiries performed with questionnaires, it appeared that several consumers choose tap water alternatives and drink bottled water for taste reasons. Taking into account these results, we set the hypothesis that the perception of chlorine flavour could be a reason why a proportion of consumers prefers bottled water to tap water. Additionally, we hypothesized that consumers who choose tap water alternatives may be more sensitive to chlorine flavour as - 16 -

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compared to consumers who accept tap water as drinking water. Therefore, we set out to determine whether chlorine flavour sensitivity could be a driver for tap water acceptability. To do so, we compared the sensitivity to chlorine flavour of a group of consumers who usually drink tap water and a group of consumers who never drink tap water. Since several forms of chlorine are used in water, and pH affects the speciation of chlorine (Bryan et al., 1973), chlorine flavour was generated from free available chlorine in Evian water at a fixed pH value. We conducted two experiments in order to determine whether tap water consumers differ from non-consumers both in chlorine sensitivity and acceptability.

The first experiment was dedicated to measure chlorine flavour detection threshold for a group of tap water consumers and a group of non-consumers. To do so, we used the constant stimuli procedure which has been recommended for its fine resolution (Wise et al.

2008). This method has the advantage of threshold estimation based on psychometric function modeling and is especially recommended for individual threshold estimations. The aim of the second experiment was to investigate putative differences between the two groups in chlorine suprathreshold sensitivity and liking. Additionally, acceptability of chlorinated water as drinking water was measured for both groups using water solutions including supra- threshold chlorine concentrations.

2. Materials and methods 2.1. Stimuli

Chlorinated water samples were obtained by adding sodium hypochlorite (NaOCl~15%, RECTAPUR, VWR international, France) to Evian water (La Bourgogne, Dijon, France).

Evian water was chosen because of its neutral taste due to medium mineral content (Teillet et al. 2008) and its compositional stability. Evian water had been also chosen in previous studies (Piriou et al., 2004; Mackey et al., 2004^a,b). Evian water was purchased in 1 L glass bottles from the same lot. In the first experiment, the concentrations of chlorine (mg/L Cl2) in the samples were the following: 0.01 mg/L, 0.03 mg/L, 0.06 mg/L, 0.1 mg/L, 0.17 mg/L, 0.32 mg/L, 1 mg/L, and 3 mg/L. In the second experiment, the concentrations of chlorine (mg/L Cl2) in the samples were the following: 0 mg/L, 0.03 mg/L, 0.1 mg/L, 0.3 mg/L, 1 mg/L, 3 mg/L, and 10 mg/L. Since sodium hypochlorite solutions provide free chlorine but also sodium ions which contribute to taste, control solutions used in discrimination tests needed to be compensated for sodium (Lugaz et al. 2002). Control solutions were prepared adding sodium chloride (NaCl, Jera, France) to reach the same sodium content as the eight chlorine solutions used in experiment 1, respectively: 0.02 mg/L, 0.05 mg/L, 0.1 mg/L, 0.17 mg/L, 0.28 mg/L, 1.65 mg/L, and 4.95 mg/L. Because of chlorine s high volatility and degradation by sunlight (UV), chlorinated solutions were prepared daily and stored until tasting, i.e., for a

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- 18 - maximum of eight hours, in brown glass 500 mL flasks equipped with brown glass stoppers.

Before use, flasks and stoppers were heated at 400°C for 2 h and then 3 times rinsed with a sodium hypochlorite solution (1%) in order to minimize chlorine demand due to residual compounds which could remain on the glass. As a consequence, the expected chlorine content of the delivered samples was controlled by free and total chorine measurement. In the absence of chlorine demand, these parameters are assumed to be close to the expected value. Both parameters were controlled daily in each flask just after solution preparation and at the end of each sensory session. These controls were performed through a procedure (pocket colorimeter II, Hach Lange) adapted from the DPD protocol for spectrophotometry (APHA-AWWA-WEF 1998). Temperature and pH of each solution were controlled (Solitrode Pt 1000 and 781 pH/Ion meter, Metrohm, Courtaboeuf, France). For sensory tasting, samples of 10 mL were delivered in plastic glasses. The absence of off-flavour due to Evian water and the cups was checked. Additionally, the absence of chemical reaction between chlorine and the plastic material was checked through free and total chlorine measurements.

2.2. Consumers

Two hundreds of consumers randomly selected from the Dijon area were first contacted for an inquiry on their water consumption and general food habits. On the basis of their responses, 72 out 200 were invited to participate to sensory deficiency screening tests and to a more extensive questionnaire on their drinking water habits. Screening tests consisted in the European test of olfactory capabilities (ETOC, Thomas-Danguin et al. 2003), a test to evaluate subject s ability to rank six chlorine solutions with increasing sodium hypochlorite concentrations and a mental concentration test (Bourdon Test, Lesschaeve 1997). This selection was done in order to avoid highly sensory deprived subjects and to be sure to include exclusive tap water consumers or exclusive tap water non-consumers in the two dedicated groups. In the end 40 out of 72 consumers participated to the study. Only one consumer was excluded because of his results to the screening tests (low score at ETOC).

The other non-selected consumers could not participate because of their non-availability during the whole testing sessions, or because of their water consumption which did not fit with the strict selection criteria. On the one hand, consumers included in the tap water consumer group were people who daily drink chlorinated tap water without using any devices or processes to reduce chlorine flavour. Following the French standard concentration, these consumers receive, at tap, a chlorine concentration between 0.1 and 0.3 mg/L. Within the Dijon area consumers receive water with a medium TDS value (305 mg/L). On the other hand, consumers included in the tap water non-consumer group were people who do not drink tap water and declared themselves to be exclusive bottled water consumers.

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The participants signed an informed consent form but the aim of the experiment was not revealed. They were asked not to modify their water consumption during the study and to avoid smoking, drinking and eating at least one hour before each session and to avoid using perfume the day of the test. Subjects were paid for their participation. Twenty tap water consumers (9 women, 11 men) with a mean age of 38±11 years and 20 tap water non- consumers (14 women, 6 men) with a mean age of 43±14 years participated in the first experiment. Only 35 of the previous 40 consumers took part in the second experiment: 18 tap water consumers (8 women, 10 men) with a mean age of 38±11 years and 17 non- consumers (11 women, 6 men) with a mean age of 41±14 years.

2.3. Experimental procedure

All experiments were performed in a room dedicated to sensory analysis following HACCP and Research Quality Insurance Standards. Consumers were placed in separate booths and their responses were collected using software dedicated to sensory analysis (FIZZ, Biosystèmes, Couternon, France). Consumers received 10 mL stimulus samples in plastic glasses coded with a three-digit number.

2.3.1. Experiment 1: chlorine flavour threshold measurement

Flavour thresholds were measured according to the constant stimuli procedure (AFNOR 2002). This procedure allows psychometric function modeling on the basis of detection probability measurement for increasing stimulus concentrations. In our experiment, the detection probability was measured using a three-alternative forced choice (3-AFC) discriminative test. In this test, three samples were simultaneously delivered to a consumer.

One of the samples contained a chlorinated stimulus, at one of the eight chlorine concentrations, and the two other samples contained the corresponding control solution (balanced for sodium content).

Subjects had to taste the three samples and to decide which one contained the chlorinated stimulus. Four repetitions were performed by each consumer at each concentration levels in order to ensure a sufficient statistical power (Schlisch 1993).

A total of 32 tests (3-AFC) were performed by each subject during two one-hour sessions (16 tests in a session; the two sessions spaced by a week). The sample presentation order was the same for all consumers; however, the location of the odd sample in one 3-AFC test was random for each trial. Subjects had to wait at least 140 s between each trial. They were instructed to rinse their mouth with Evian water during this inter-trial interval.

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2.3.2. Experiment 2: chlorine flavour intensity, liking and acceptability rating

Chlorine flavour intensity, liking and acceptability were evaluated within a third session (lasting 1.5 h). The session was divided into two blocks; the first one was dedicated to liking and acceptability ratings and the second to intensity rating.

The sample presentation order was randomized and different in each block. Four different presentation orders were used within a block. Within the first block, liking for each sample was rated on a 23 cm linear scale from I don t like this sample to I like this sample (Fig.

1). Ratings on this scale were normalized to obtain a score between 0 and 10. For acceptability, consumers were asked If this water was daily delivered to your tap, would you drink it? They were instructed to answer by yes or no (Fig. 1). Within the second block, consumers rated chlorine flavour intensity on a 23 cm linear scale from It doesn t taste chlorine to It strongly tastes chlorine (Fig. 1). Ratings on this scale were normalized to obtain a score between 0 and 10. Subjects had to wait at least 140 s between each sample.

They were instructed to rinse their mouth with Evian water during this inter-trial interval.

2.4. Data analysis

All statistical analyses were performed using SAS release 9.1.3 (SAS Institute Inc., Cary, NC). Sensory data analyses were based on mixed modelling procedures which allow considering subjects as a random factor. This means that it could be considered that the consumers included in the tap water consumer group (respectively the non-consumer group) were randomly selected within the tap water consumer population (respectively the non- consumer population) and could be considered as representative of the French tap water consumer population (respectively the non-consumer population; Pinheiro and Bates 2000).

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2.4.1. Experiment 1

In the 3-AFC tests, correct answers were coded with a 1 and incorrect with a 0. Each subject performed 4 repetitions (n = 4) at each concentration. The number of correct answers Y is distributed as a binomial law [B(n,p)] where the correct answer probability p combines correct answers occurring by chance (i.e., 1/3) and correct answers due to real detection (pd, equation (1)).

(1) 1

(1 )

d 3 d

pp  p

pd, the detection probability, follows a sigmoid function of the logarithm of the stimulus concentration (psychometric function). Therefore, the logit function of pd is assumed to be linearly linked to the logarithm of the stimulus concentration (equation (2)).

(2)

( ) ln( ) log (

¹⁰

)

1

d d

d

Logit p p concentration

p  

  



In equation (2), the slope is assumed to be constant whereas the intercept varies according to the subject sensitivity. Indeed, since each subject s sensitivity to the stimulus is different, the constant term ( ) was supposed to be random. Therefore, we introduced for each subject a constant term ( s) randomly distributed according to the normal law around the average value  (equation (3)).

(3) y ~ B(n,p) with p such as:

  

  

¹⁰

1 log( 3 ) log ( )

1

^s

p

concentration

p

^and

^s^{~ N(}^,^s^²⁾

By definition, the threshold is the concentration detected with a probability equal to 50%, that is, pd = 0.5 (AFNOR 2002). Using equation (3), the threshold could be estimated for each group of consumers (equation (4)) but also for each subject (equation (5)) even if some of them did not detect 100% of the trials at the highest concentration level.

(4)

log (

10

Threshold ) 

  

(5)

log (

10

Threshold ) 

^s

  

The modeling was performed using SAS and the parameters were estimated using the NLMIXED procedure. A first model (model 1) was used to estimate the mean threshold for the whole set of consumers. Additionally, and since we hypothesized that the two consumer groups might differ in their chlorine flavour thresholds, a second model (model 2) was also

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- 22 - implemented to take into account this putative sensitivity difference. In this model, the term is assumed to be different between the two groups and could have a different value ( TWC for tap water consumers and or TWNC for non-consumers). The second model differs from the first one only in the ^s term and can be written ^s follows:

^s~ N(^TWC+, ^s^²)

Thus, ^TWC is used to take into account the sensitivity of tap water consumers and ∆ to account for a putative sensitivity difference between both the consumer groups. As a consequence, ^TWNC is the sum of ^TWCand ∆.

Therefore, testing the difference between both the groups means testing the significance of

∆. This can be achieved by comparison of the two models (model 1 and model 2), through a nested model comparison using likelihood ratio tests. This comparison aimed to determine whether model 2 better fits the data than model 1. In other words, this comparison allowed us to determine whether the sensory difference between tap water consumers and non- consumers is statistically significant.

2.4.2. Experiment 2

Intensity and liking rating data were submitted to an ANCOVA (Analysis of Covariance) with subjects as random factor and chlorine concentration as covariate (GLM procedure of SAS).

Post hoc comparisons of means were performed with a Bonferroni adjustment for multiple comparisons. Acceptability data (yes or no answers) were analyzed through the Generalized Equation Estimation for binary data (GENMOD procedure of SAS) with subjects as a repeated effect (Zeger et al. 1988).

3. Results

Free and total chlorine contents of the samples delivered to the panelists were controlled and physico-chemical data confirmed that, for both experiments, each solution contained the expected amount of free and total chlorine (Table 1).

Multiple comparisons of means revealed that chlorine concentration differed significantly between each level with the exception of the two first levels (Table 1). Solutions had a mean pH value of 7.54±0.01 and the mean temperature of the samples was of 21.1±0.1°C.

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3.1. Experiment 1: chlorine flavour threshold measurement

The objective of this experiment was to determine the chlorine flavour detection threshold for a group of tap water consumers and for a group of non-consumers. We especially wanted to determine whether these two groups had a different mean sensitivity toward chlorine flavour.

Individual psychometric functions were recorded for each panelist. An example of this function is presented in Fig. 2.

Data analysis recommended by the International Organization for Standardization for sensitivity thresholds measurement (AFNOR 2002) is based on a modeling of individual psychometric functions leading to individual threshold estimation. This was done following a global modeling approach in which (i) the number of correct answers is assumed to follow a binomial law, (ii) the logit of the probability of detection is assumed to be linearly related to the logarithm of the concentration, and (iii) each subject s sensitivity to the stimulus is

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- 24 - assumed to be randomly distributed, according to a normal distribution. Two models were actually compared: the first one was used to estimate the mean threshold for the whole set of consumers whereas the second one took into account a putative sensitivity difference between the two groups of panelists. The estimated thresholds for the tap water consumer group were 0.21 mg/L (SEM= 0.16) and 0.09 mg/L for the non-consumer group (SEM= 0.07).

However, the comparison (nested model likelihood ratio test) revealed no significant difference ( ² (1, N=40) = 0.6, p = 0.44) between the two models, which demonstrated that the data were equally well-described by both models. As a consequence, chlorine detection threshold had to be estimated using the first model which implies that the difference between detection thresholds of both groups should not be considered to be significantly different.

Therefore, the mean chlorine flavour detection threshold estimated through the first model was 0.14 mg/L (SEM= 0.08) for the whole set of consumers. Individual thresholds were also estimated. Their distribution indicated that 65% of the consumers had a chlorine detection threshold lower than 0.3 mg/L Cl2 (Fig. 3) which corresponds to the French standard concentration to be fixed at the exit of the treatment plant. It is noteworthy that the dispersion of chlorine flavour detection thresholds was found to be large.

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3.2. Experiment 2: chlorine flavour intensity, liking and acceptability rating

The second experiment was dedicated to supra-threshold chlorinated water flavour intensity, liking and acceptability assessments. Data were especially analyzed to test differences between tap water consumers and non-consumers. A two-way ANCOVA (subject within group, group and chlorine concentration as covariate) was performed on both flavour intensity and liking and revealed a significant effect of the chlorine concentration on both intensity (F(1,204)= 77; p<0.0001) and liking (F(1,209) = 38; p<0.0001). Fig. 4 shows that intensity ratings increased with concentration. Surprisingly, the stimulus response curve did not follow the theoretical stimulus response function (Chastrette et al. 1998). Intensity increased from 0 to 0.3 mg/L Cl2, then, reached a plateau and followed a final upward trend from 1 mg/L Cl2 up to 10 mg/L. As far as hedonic rating is concerned, the liking was found to decrease when the stimulus concentration increased with a plateau between 0.1 and 3 mg/L Cl2. ANCOVA results indicated that tap water consumers were not different from non-consumers as far as chlorine flavour intensity rating was concerned (group: F(1,33)=1.3; p=0.26). However, tap water non-consumers expressed a lower liking for chlorinated solutions as compared to regular consumers (group: F(1,33)= 4.0; p=0.05) (Fig. 5). Generalized equation estimation for binary data was applied to analyze acceptability data. Consumer group and chlorine concentration were tested as factors. A significant effect of chlorine concentration was found on acceptability (z(1,242)=3.93; p<0.0001). Chlorine flavour acceptance decreased with increasing chlorine concentration and followed the same pattern as hedonic ratings (Fig. 6).

In addition, a highly significant effect of the group of consumer on acceptability was noticed (z(1,242)=3.1; p=0.002). Tap water consumers declared being more inclined to consume the chlorinated water samples delivered in this experiment than tap water non-consumers.

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4. Discussion

First of all, our recruitment procedure as well as our data analyses allow to consider participants included in consumer groups as a random factor. Therefore, it could be considered that the consumers included in the tap water consumer group (respectively the non-consumer group) were representative of the exclusive tap water consumer population (respectively the non-consumer population). Consequently, our results should not be restricted to the community of the people tested but could be extended to the two populations of French consumers.

Our results did not provide evidence of any difference in chlorine flavour sensitivity, at the detection threshold level, between tap water consumers and non-consumers. The detection threshold for both groups was 0.14 mg/L which is consistent with previously published thresholds: 0.16 mg/L Cl2 (Bryan et al., 1973), 0.24 mg/L Cl2 (Krasner and Barrett, 1984) and 0.2 mg/L Cl2 (Piriou et al., 2004). We also observed a large inter-individual difference in

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chlorine flavour sensitivity. These results are especially in agreement with those obtained by Mackey et al. (2004^a,b), who studied public perceptions of chlorine flavour in drinking water and their impact on customer s choices with respect to drinking water consumption habits.

Indeed, these authors did not find a significant difference between tap water consumers and users of tap water alternatives but noticed a large inter-individual difference in sensitivity. We did not find a significant difference between the two groups of consumers for chlorine flavour supra-threshold intensity perception. However, we observed a difference between tap water consumers and non-consumers for liking and acceptability of chlorinated solutions. Beyond inter-individual differences, it is important to notice that intensity relies on different perceptual processes as compared to hedonic perception (Bensafi et al. 2003). Indeed, liking and intensity judgment elicit activation in similar brain areas but pleasantness also elicits activation in the hypothalamus, which is known to be involved in affective processing which requires access to information about internal states (Zatorre et al. 2000). In other words, pleasantness judgments imply a decision: in this case, whether one odour might indicate something good to eat or something that could make one sick.

It is thus not surprising that, for similar flavour intensity levels, tap water non-consumers appreciated chlorinated solutions less than tap water consumers. This was confirmed by our findings, which demonstrated that tap water non-consumers accepted less chlorinated solutions as water to be drunk than tap water consumers. It is likely that acceptability judgments rely on high cognitive aspects, since consumers were asked to choose if they would drink the chlorinated solutions in the context of daily tap water consumption. Whether consumer sensitivity and perception could be a driver for food liking and acceptability remains an unanswered issue. For example, regular caffeine users were found to have higher detection thresholds for caffeine than non-users (Tanimura and Mattes 1993). In contrast, Delahunty and Lee (2007) evidenced that acceptance and fruit consumption were related to sweet and sour liking but not to sensitivity toward these tastes. Beside sensitivity, many factors have to be taken into account to understand food choice. Hudon et al. (1991) who investigated water consumption habits in Canada, through a consumer telephone survey, noticed a group of consumers whose tap water representation was quite positive with no indication of any health risk. These consumers also declared to drink tap water and found it to be of good organoleptic, chemical and bacteriological quality. Conversely, another group of consumers, whose tap water representation was clearly negative on the same items, declared they drank bottled water. This link between tap water representation, safety and organoleptic properties was also suggested by McGuire (1995): consumers who detect off- flavours in their drinking water likely associate these perceptions to a lack of water safety, even if there is no link between organoleptic effects of drinking water contaminants and their actual toxicity (Young et al. 1996). Torobin et al. (1999) also noticed a correlation between

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- 28 - perceived safety and actual water taste and suggested that taste does shape safety perception. Our data showed that increased chlorine flavour perception (intensity) was associated with a more pronounced decrease of water acceptability in tap water non- consumers. As a result, chlorine flavour seems to play in fine against tap water acceptability as a marker of safety representation. It is nevertheless noteworthy that other extrinsic factors such as environmental perception (e.g., raw water bad quality) have also been highlighted to influence consumer choice (Bontemps and Nauges 2006). Our data suggested that most of French consumers perceive chlorine flavour at tap since a decree of the French Public Health Authority imposes a minimal value of 0.3 mg/L Cl2 at the treatment plant outlet and a minimum of 0.1 mg/L at tap (Ministère de la santé, de la famille et des personnes handicapées, 2001). Indeed our data predicted that 65% of the consumers may have a threshold below 0.3 mg/L Cl2 (mean threshold value at 0.14 mg/L Cl2). Once perceived, chlorine flavour may constitute a marker of water acceptability as a beverage depending on individual tap water representation. Tap water consumption could be seen as the result of a subtle balance between sensitivity, actual chlorine content of tap water and tap water representation. The two last parameters are susceptible to be modified through water supply process improvement or tap water information delivery. Another outcome of the present study is the non-classical stimulus response function observed for chlorine flavour intensity.

Indeed, it is commonly admitted in chemosensory perception that the theoretical stimulus response function should follow a monotonically increasing sigmoid shape from the threshold plateau to the saturation plateau seldom reached at high stimulus concentrations (Chastrette et al., 1998). However, our data showed that chlorine flavour intensity increased from 0 to 0.3 mg/L then reached a plateau until 1 mg/L and finally followed a final upward trend up to 10 mg/L.

This observation seems to be consistent as the same profiles were observed for liking and acceptability measurements, also showing a plateau in this range of concentrations. The physico-chemical controls performed confirmed that these observations cannot be due to errors in chlorinated solution preparation, neither to temperature differences between samples (Whelton and Dietrich 2004) nor to solution conservation problems. One possible explanation would rather be found in the fact that chlorine flavour is an integrated perception which potentially combines several sensory modalities (Thomas-Danguin 2009; Auvray and Spence 2008; Small and Prescott 2005). Thus, the non-classical shape of the stimulus response curve could be explained by the implication of at least two sensory modalities in the chlorine flavour perception. A first sensory mechanism would be involved until the first plateau (first saturation mechanism between 0.3 and 1 mg/L) followed by a second sensory mechanism. In previous studies, chlorine flavour perception was described through Flavour Profile Analysis (FPA) method using a rather limited range of concentrations (Krasner and

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Barrett, 1984) and suggested that chlorine flavour included an olfactory dimension. However, sensory mechanisms implicated in chlorine perception remained largely unknown. Gustatory and trigeminal systems could likely be involved in chlorine flavour perception. Indeed, most odorants also activate the trigeminal nerve and trigeminal thresholds are usually higher than olfactory ones. Therefore, the second part of the observed stimulus response curve at concentrations higher than 1 mg/L Cl2 might be reconcilable with trigeminal system activation. It is noteworthy that following such a hypothesis, these high chlorine concentrations also correspond to an important drop in liking and acceptability which suggests that the second sensory mechanism activated would have a great role in chlorinated tap water lack of acceptability. Nevertheless, this hypothesis remains to be confirmed by further investigations.

5. Conclusion

In the present study, regular tap water consumers were found to be as sensitive to chlorine flavour as bottled water consumers. However, bottled water consumers showed a lower appreciation of chlorinated water solutions and were especially less inclined to accept chlorinated water as drinking water delivered at the tap. These results highlight the importance of tap water representation, beyond chlorine sensitivity and flavour perception, in water consumption and choices. Since very different participants were tested in terms of their water consumption habits, these findings may be extended to the French population.

However, it would be interesting to perform similar studies in other countries in order to evaluate the influence of chlorine exposition level and other cognitive factors.

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- 30 -

3. Publication 2:

Trigeminal perception is involved in chlorine flavour perception and could account for Tap water rejection

Sabine PUGET^1,2, Noëlle BENO², Claire CHABANET², Elisabeth GUICHARD², Thierry THOMAS- DANGUIN^2,*

1 Lyonnaise des Eaux, 11 Place Edouard VII, F-75009 Paris, France.

2 Unité Mixte de Recherches FLAVIC, INRA, ENESAD, Université de Bourgogne, 17 rue Sully, BP 86510, 21065 Dijon Cedex, France.

* Corresponding author: Tel: +33 380693084; fax: +33 380693227; e-mail: Thierry.Thomas- [email protected].

1. Introduction

Chlorine flavour constitutes one of the major complaints addressed against tap water (Chotard 2008; Suffet et al. 1996; Hudon et al. 1991). Nevertheless, chlorine addition is a necessity for tap water suppliers to maintain a high bacteriological quality throughout the water-supply network. Most often, chlorination treatment is ensured by gaseous chlorine addition in water (Connell 1996). Chlorine¹ (Cl2) immediately reacts with water to form hypochlorous acid which is present both in its associated form (HOCl) and dissociated form (ClO^-) at the tap water pH value (6-8; Doré 1989). pKa of this acid is 7.5. Below this value, the quantity of HOCl increases whereas the quantity of ClO^- decreases.

1 At the treatment plant, chlorine gas (Cl2) immediately reacts with water to form hypochlorous acid. As a consequence, the term chlorine cannot be used for the stimulus as far as chlorine in water is concerned. In this publication, the term chlorine flavour will be used only to describe the perception elicited by hypochlorous acid and it will be mentioned whether it is the associated (HOCl) or dissociated form (ClO^-).

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Bryan, Kuzminiski et al. (1973) who measured chlorine flavour thresholds showed that chlorine flavour threshold decreases according to pH. Thus, an increase of the volatile associated form (HOCl) is associated with an apparent threshold decrease. However, this cannot be attributed to an increase in sensitivity since the mechanism only relies on the increase in stimulus concentration. Nevertheless, these data suggest that the volatile associated form, i.e. HOCl, is responsible of the sensory perception. Perception of chlorine flavour in water has rarely been studied in depth. Mackey et al. (2004a) have studied public perception of chlorine flavour in drinking water. They did not find a link between chlorine flavour thresholds and tap water consumption habits. In a previous study (Puget et al. 2010), we consolidated this result, since we did not find a significant difference in chlorine flavour perception (i.e. detection threshold and intensity) between tap water consumers and non- consumers. However, we evidenced a significant difference between these two groups in terms of preference and acceptability for solutions realised with increasing chlorine concentrations. These findings have suggested that, beyond chlorine sensitivity and flavour perception, tap water representations may account for water consumption behaviours and choices. In the same study (Puget, Beno et al. 2010), we also observed a relatively atypical profile of the stimulus-response curve. Perceived odour intensity usually increases as a monotonic function of the chemical stimulus concentration (Chastrette, Thomas-Danguin et al. 1998). In contrast, chlorine flavour intensity was found to increase between the detection threshold (0.14 mg/L free chlorine Cl2 equivalent²) and 0.3 mg/L Cl2; a plateau was observed between 0.3 and 3 mg/L Cl2 then a sharp increase in intensity was noted between 3 and 10 mg/L Cl2 (Puget, Beno et al. 2010). These results suggested that at least two sensory mechanisms could be involved in chlorine flavour perception. Indeed, Flavour is defined as a sensory percept which relies on the functional integration of information transmitted by the chemical senses: olfaction, gustation, oral and nasal somatosensory inputs (Small and Prescott 2005; Hummel 2008; Verhagen and Engelen 2006).

The mechanisms implied in chlorine flavour perception remain largely unexplored. If HOCl is likely the actual chemical stimulus as suggested by the data reported by Bryan, Kuzminiski et

2 In this publication such as in the whole manuscript, concentration of hypochlorous acid in water are free chlorine expressed in mg/L Cl2 equivalent. This will be written mg/L Cl2.

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- 32 - al. (1973), it is not known whether volatile HOCl could activate both olfactory and nasal trigeminal systems. Additionally, once in the mouth, chlorine in water can activate both gustatory and oral trigeminal systems. Even if no difference in sensitivity could be observed previously between tap water consumers and non-consumers for chlorine flavour threshold, it cannot be excluded that differences in one chemical sense sensitivity exists and can therefore account for variations in preferences and acceptability observed in relation to consumption habits.

It is possible to perform independent measurements of the olfactory, gustatory and somatosensory activation threshold. Phenomena occurring in nose or in mouth can be easily distinguished respectively through retronasal tasting with or without a nose clip. In fact, the use of nose clip prevents air flow movement conveying volatile compounds retronasally to the olfactory epithelium. Once in the mouth or in the nose, it is more difficult to distinguish taste and smell from trigeminal perception.

Indeed, trigeminal and gustatory systems are intimately associated in mouth. Most of the fibers entering the fungiform papillae are trigeminal (Farbman and Hellekant 1978) and interactions between gustatory and somatosensory nerve exist from the periphery to the brain (Faurion 2004). As a consequence, it is not straightforward to distinguish between the taste and the chemosensory perception elicited by a compound. Conversely, several methods were proposed to distinguish between odour and nasal irritation perception. In 2002, Shusterman reviewed the different methods developed to assess trigeminal function depending on whether physiological or perceptual responses are registered. The authors especially reviewed methods distinguishing between odour and nasal irritation perception.

The first method consisted to ask subjects to rate irritation using specific instructions that focalise subjects attention on trigeminal sensations. However, this method is sensitive to confusion between odour and irritation and has been judged less objective. Indeed, subjects responses could integrate cognitive aspects of the perception (Frasnelli and Hummel 2005).

Other methods compared nasal detection threshold in normosmic and in anosmic subjects.

Detection threshold obtained with a normosmic panel is assumed to be the olfactory threshold whereas detection threshold measured in anosmics is supposed to be the trigeminal threshold. These studies have been especially used to determine properties of irritants such as the ability of the trigeminal system to detect series of chemicals (Cometto- Muniz et al. 2005; Cometto-Muniz et al. 1998; Cometto-Muniz et al. 1999). Anosmic subjects were found to be less sensitive than normosmic ones to trigeminal stimulation due to the lack of central nervous interaction between these two sensory systems (Frasnelli and Hummel 2007; Hummel et al. 2003). Iannilli et al. (2007) also demonstrated that activation pattern in response to a trigeminal compound such as CO2 were different in both populations.

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A third methodology based on nasal lateralization thresholds is more and more often used to estimate trigeminal sensitivity. Indeed trigeminal thresholds can be inferred from lateralization thresholds obtained from individuals with normal olfactory capability. The lateralization procedure relies on the possibility for human subjects to localize the nostril in which a trigeminal volatile compound is delivered. Such localization is not possible for pure olfactory compounds or when the odorant concentration is too low to activate the trigeminal system (Kobal et al. 1989). This method which has been validated through studies including electrophysiological measurements (Cometto-Muniz and Cain 1997; Stuck et al. 2006;

Wysocki et al. 2003) was the one retained in the present experiment to estimate trigeminal thresholds.

Several published data (e.g. Doty 1975; Doty et al. 1978) indicated that olfaction is most often activated at lower chemical stimulus concentrations as compared to the trigeminal system. Such a difference in activation thresholds could account for an atypical profile of the stimulus-response curve. Indeed, the activation of the olfactory system at low concentration could lead to an increase of perceived intensity when concentration increases. At higher concentrations, the activation of the trigeminal system could lead to a modification of the perceived stimulus intensity, which may result in a modification of the stimulus-response curve slope. In this paper, we tested this hypothesis and set out to measure olfactory and trigeminal detection thresholds for chlorinated water solutions. We also measured an in mouth detection score at a high chlorine concentration. This last measure especially aimed to assess the putative activation of the gustatory system. All measurements were performed for a group of tap water consumers and a group of non-consumers since we wanted to know whether sensitivity differences in one of the chemical senses could be evidenced and therefore account for variations in preferences and acceptability observed in relation to tap water consumption habits (Puget, Beno et al. 2010).

2. Materials and methods

All experiments were performed in a room dedicated to olfactometry following HACCP and Research Quality Insurance Standards. Consumers responses were collected using software dedicated to sensory analysis (FIZZ, Biosystèmes, Couternon, France).

2.1. Consumers

Two hundreds consumers randomly selected from the Dijon area were first contacted for an inquiry on their water consumption and general food habits. On the basis of their responses, 72 out 200 were invited to participate to sensory deficiency screening tests and to a more extensive questionnaire on their drinking water habits. Screening tests consisted in the European test of olfactory capabilities (ETOC, Thomas-Danguin et al., 2003), a test to

Chlorine flavor perception and neutralization in drinking water

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Chlorine flavor perception and neutralization in drinking water

Sabine Puget

To cite this version:

Sciences de l alimentation

Présentée par Sabine Puget

Chlorine flavour perception and neutralization in drinking water

Directeur de thèse : Elisabeth GUICHARD

Co-directeur de thèse : Thierry Thomas-Danguin

Acknowledgments

Table of contents

General introduction

Chapter 1.

Chlorine flavour perception

1. Introduction

2. Publication 1:

Tap water consumers differ from non-consumers in chlorine flavour acceptability but not sensitivity.

1. Introduction

2. Materials and methods 2.1. Stimuli

2.2. Consumers

2.3. Experimental procedure

2.3.1. Experiment 1: chlorine flavour threshold measurement

2.3.2. Experiment 2: chlorine flavour intensity, liking and acceptability rating

2.4. Data analysis

2.4.1. Experiment 1

( ) ln( ) log (

)

1

Logit p p concentration

p  

  



  

  

1

log( 3 ) log ( )

1

p

concentration

p

log (

Threshold ) 

  

log (

Threshold ) 

  

2.4.2. Experiment 2

3. Results

3.1. Experiment 1: chlorine flavour threshold measurement

3.2. Experiment 2: chlorine flavour intensity, liking and acceptability rating

4. Discussion

5. Conclusion

3. Publication 2:

Trigeminal perception is involved in chlorine flavour perception and could account for Tap water rejection

1. Introduction

2. Materials and methods

2.1. Consumers